(1) Field of the Invention
The present invention relates to a toner for the full color development where a plurality of toners are overlapped on an image on a copying sheet. More particularly, the present invention relates to a toner for the full color development, in which development characteristics and transfer characteristics are made substantially equal in the toners to be overlapped.
Furthermore, the present invention relates to magenta, cyan and yellow toners among toners for the full color development. More particularly, the present invention relates to these toners having such an excellent transparency that when these toners are mingled on an image on a transfer sheet, the respective toners can show intended colors sharply.
(2) Description of the Related Art
In the fields of the electrophotography and electrostatic printing, toners are used for visualizing electrostatic latent images formed on image carriers. In these toners, a resin having desirable electroscopic and binding properties, for example, a styrene resin or a polyester resin, is used as the resin medium, and carbon black or other organic or inorganic coloring pigment is used as the coloring agent.
The full color development in which magenta, cyan, yellow and black color toners are overlapped to form an image has been recently proposed and worked.
In this full color development, a multiple-color original is exposed to light through a color-separating filter, this operation is repeated a plurality of times by using cyan, yellow and magenta color developers and a black toner, and toner images are thus overlapped to obtain a multiple-color image. Organic pigments are used as coloring agents for cyan, yellow and magenta toners used for this full color development, and carbon black is used for a black toner.
FIG. 7 is a diagram illustrating developing and transfer zones of an image-forming apparatus for obtaining a full color image. In this apparatus, an electrostatic latent image formed on a photosensitive drum 1 by appropriate means is visualized by a developer in any of developing devices 3a, 3b, 3c and 3d of a developing unit 2 and is then transferred by a transfer charger 5 onto a transfer material held on a transfer drum 4 by a gripper 6, from which electricity is removed by an electricity-removing charger 7. Furthermore, a toner image developed by a developer in another developing device of the devices 3a, 3b, 3c and 3d is transferred onto the transfer material by the transfer charger, and third and fourth color images are similarly transferred. Thus, a predetermined number of color images are transferred onto the transfer material held on the transfer drum 4, and the transfer material is delivered to a fixing step (not shown) to form a multiple-color image. In general, in the above-mentioned transfer step, an operation of transferring a toner of a different color onto a toner layer transferred on a transfer material is carried out. At this operation, it sometimes happens that the charge of the toner already transferred on the transfer material reduces the working transfer electric field at the transfer of the subsequent toner and therefore, an image having a desired hue cannot be reproduced. For obviating this disadvantage, there is sometimes adopted a method in which the transfer voltage is gradually elevated at the transfer step or the transfer voltage is elevated at the transfer of the third and subsequent toners where the toner layer becomes thick.
However, since behaviors of toners at the practical transfer step are delicate and complicated, even if a predetermined transfer voltage is applied and the value of the transfer voltage is elevated in the later stage, scattering of the toners or insufficient transfer is often caused because the respective color toners are different in various characteristics (such as charging characteristics and electric characteristics), and no satisfactory results can be obtained in formation of a toner image of a desirable hue.
Japanese Unexamined Patent Publication No. 01-32981 proposes a method in which the quantity of the charge of a toner to be developed and transferred is made larger than the absolute value of the already developed and transferred toner to compensate the reduction of the working transfer electric field and stabilize the transfer operation. According to this method, if it is intended to adopt common development conditions (charge characteristics of the photosensitive material, the development bias voltage and the sliding contact state between the photosensitive material and the developer carrier), since toners are extremely different in the charge characteristics, development unevenness (Insufficient density of the solid portion, thickening of line and dot images and formation of toner dusts in the peripheral portion of the image area) is caused or scattering of toners is caused in the machine, and a shear in the hue and a fog are often observed in the formed image.
It is important that color toners should be excellent not only in spectral reflection characteristics but also in spectral transmission characteristics, and if this requirement is not satisfied, an image having a hue similar to the inherent color cannot be obtained. When a full color image is formed by overlapping a plurality of color toners, it is especially important that a transparency should be given to the toners. If color toners poor in the transparency are used, the colors of the toners interfere with one another and the formed image becomes dark, and it often happens that an image of a desired color cannot be obtained.
As the means for overcoming the foregoing defects, there have been proposed a method in which a specific fluorine-containing acrylic resin is used as a binder resin medium (Japanese Unexamined Patent Publication No. 62-273569) and a method in which an oil-soluble dye such as C.I. Solvent Yellow 60 is incorporated into a yellow toner (Japanese Unexamined Patent Publication No. 62-273572).
However, even if these methods are adopted, the original image hue cannot be sharply reproduced by mingling the colors, and it often happens that the formed image becomes obscure and the characteristics of colors are not effectively utilized. Therefore, the problem cannot be solved by these methods.